This invention relates to reinforced silicone elastomers. More particularly, it relates to finely divided silica fillers, useful in reinforcing fluorosilicone elastomeric compositions which have been treated with pre-hydrolyzed fluoroalkyl-functional diorganodihalogensilanes. Fluorosilicone elastomers containing silica fillers so treated show substantially improved compression set and tear strength, as well as improved handling properties.
Silicone elastomers have been widely valued for their resistance to moisture and their high and low temperature stability. Improved silicone elastomers have also been developed which exhibit better handling characteristics, as in U.S. Pat. No. 2,938,009 (Lucas), mechanical properties, as in U.S. Pat. Nos. 3,635,743 (Smith) and 3,847,848 (Beers), and solvent resistance, as in U.S. Pat. No. 4,029,629 (Jeram). All of the above patents are incorporated herein by reference. These improvements are accomplished through the use of treated fillers, usually treated finely divided silica, or by modifying the curable siloxane polymers, as with the perfluoroalkylene-substituted polysiloxanes of the aforementioned Jeram patent.
Treating silica fillers with fluorosilicone treating agents has been proposed, see e.g., copending U.S. Applications Ser. No. 252,659 filed Apr. 9, 1981, now U.S. Pat. No. 4,355,121 and Ser. No. 195,579 filed Nov. 8, 1980, now abandoned in favor of Ser. No. 368,931 filed April 16, 1982, but implementation of the concept has been resisted because of the toxicity of the treating agents (i.e., fluoroalkyl functional cyclic polysiloxanes) and the substantial modifications of existing equipment their use would entail.
It has now been discovered that finely divided silica reinforcing fillers can be treated with fluoroalkyl-functional silicone polymers without modifying conventional production apparatus and without adding costly toxicity controls. Furthermore, fluorosilicone elastomers reinforced with these fillers show improved ease of handling in mixing, milling and extrusion and also have improved mechanical properties, especially in terms of tear strength and compression set.